1. Field of the Invention
The present invention relates to a voltage generating circuit comprising a voltage converter which inputs a reference voltage thereto and voltage-converting the inputted reference voltage, and an output unit which impedance-converts the voltage outputted from the voltage converter and outputs the converted voltage, more particularly to a technology for realizing such drive performance that achieve a high speed and a low impedance. The present invention is suitably applied, for example, to a liquid crystal display device wherein a liquid crystal display driver, a control circuit, RAM, and the like are mixedly provided.
2. Description of the Related Art
The following constitutions are conventionally known in an operational amplifier: a circuit configuration wherein a rail-to-rail operational amplifier is adopted to amplify an entire range of power supply voltages; and a configuration which realize a voltage generating circuit capable of outputting a grayscale voltage suitable for liquid crystal display at low power consumption and in a small area by dividing a circuit system for each voltage (for example, see 2006-318381 of the Japanese Patent Applications Laid-Open). Also known are a rail-to-rail operational amplifier, a gain-boost operational amplifier and a rail-to-rail operational amplifier which also serves as a gain-boost type at the same time. A rail-to-rail operational amplifier means an operational amplifier which is operable in the whole range of the power supply voltages and driven at a high speed. An operable range of an operational amplifier of this type includes a whole range of voltages between a high-voltage-side power supply voltage VDD and a low-voltage-side power supply voltage VSS.
In a mobile device, a typical example of which is a mobile telephone, and a car navigation system, lower voltages are increasingly used as a power supply voltage because it is demanded that power consumption in terminals and devices be desirably reduced, and dielectric breakdown voltages in transistors, such as a digital circuit and RAM, are dropping as a gate oxide film becomes thinner along with the miniaturization of a semiconductor process.
Further, in a liquid crystal display driver, although there has been a trend that a power supply voltage for the driver is lowered as the display device attains a higher definition and a larger screen, there is a market demand that the grayscale voltage should have a wider range and be set high so as to improve image brightness.
A circuit which generates a grayscale voltage generates a voltage suitable for the liquid crystal display by dividing resistances and using band gap reference, switched capacitor, or the like. More specifically, the grayscale voltage generating circuit voltage-converts a reference voltage inputted to a voltage converter (differential amplifier), and impedance-converts the resulting voltage in an output unit and then outputs it to thereby generate one grayscale voltage. In the case of multi-level grayscales, a plurality of amplifiers are provided so that a plurality of voltages suitable for the liquid crystal display are generated.
FIG. 9 is a drawing schematically illustrating a conventional voltage generating circuit, and FIG. 10 is a circuit diagram which more specifically illustrates the circuit. Referring to reference numerals shown therein, 10 denotes a voltage converter which voltage-converts a reference voltage VIN, and 20 denotes an output unit which impedance-converts the voltage outputted from the voltage converter 10 and outputs the resultant voltage. The voltage converter 10 comprises an operational amplifier OP1 which is a differential amplifier. A voltage of a high-voltage-side power supply of the operational amplifier OP1 is Va, and a voltage of a low-voltage-side power supply thereof is Vb. The output unit 20 comprises an operational amplifier OP2 which is a voltage follower. A voltage of a high-voltage-side power supply of the operational amplifier OP2 is also Va, and a voltage of a low-voltage-side power supply thereof is also Vb. Vc denotes a ground level.
The voltage generating circuit inputs the reference voltage as the input voltage VIN, and then generates the grayscale voltage for the liquid crystal display as an output voltage VOUT based on image data for the liquid crystal display, and supplies the generated output voltage VOUT to a γ generating circuit in a source driver (data line driving circuit) and an output buffer at each terminal of the source driver. In recent years, with regard to the output voltage VOUT (grayscale voltage) of the voltage generating circuit, a higher voltage is demanded as the brightness and the grayscale in the liquid crystal display device are increasingly advanced, while the voltage is reduced particularly when it is used as a power supply voltage in a mobile telephone.
The voltage converter 10 generates the input voltage VIN which ranges from the power supply voltage Va on the high-voltage side through the power supply voltage Vb on the low-voltage side by subjecting the input voltage VIN to resistance division processing and non-inversion amplification processing, and then generates the grayscale voltage suitable for the liquid crystal display based on the input voltage VIN. The output unit 20 impedance-converts the voltage generated in the voltage converter 10 and outputs the resultant voltage. The voltage is impedance-converted because loads are driven at a high speed when large input impedances are converted into small output impedances.
In the conventional voltage generating circuit, in order to satisfy two demands contradictory to each other: the reduction of the power supply voltage and the increase of the grayscale voltage, the power supply voltage is limitedly reduced within a certain range, the range of the grayscale voltage are broadened, and the rail-to-rail operational amplifier is adopted so as to efficiently utilize the given voltage range.
The method illustrated in FIGS. 9 and 10, however, has the following three problems. The first problem is that the power reduction is unfavorably limited because the power supply voltage can only be reduced within a certain range.
The second problem is as described below. The grayscale voltage is given to the γ generating circuit and the buffers in the source driver serving as the load. The γ generating circuit is often configured to have several-ten kΩ order resistances. In order to drive the voltage generating circuit thus constituted, it is necessary to supply the current obtained when the grayscale voltage is divided by several tens of k Ω. Further, when the buffers in the source drive are connected, input capacitance as much as the number of channels serves as a load in the case where all of the buffers are connected, which often results in the nF order resistance. When such a load as relatively large resistance or capacitance is driven, it is difficult to drive it at a high speed by a voltage equal to or near the power supply voltage even though the rail-to-rail operational amplifier is used. In the case where resistance is a load, the voltage is divided between the resistance as the load and the resistance in the γ generating circuit unless the output impedance is reduced, which causes an error in an output voltage to occur.
The third problem is that it is difficult even for the rail-to-rail operational amplifier to drive voltages in the voltage range including the power supply voltage and voltages near the power supply voltage because the output transistor is operated in a non-saturation region, which changes the output impedance, and as a result, an error in the output voltage occurs.